This invention relates generally to blow molding machines and particularly to a machine which has special utility in the blow molding of large hollow articles. For example, it is particularly well suited for the blow molding of large cylindrically walled drums, such as commonly referred to as 55 gallon drums.
Blow molding machines are utilized to blow mold hollow plastic articles, and it has heretofore been known to blow mold articles of almost innumerable shapes with a wide variety of different types of blow molding machines. Blow molding machines may comprise multiple stations at which different operations of the overall blow molding process are conducted. Hence, a blow molded article is created by successively advancing mold cavities through the various stations until the completed articles are finally removed.
A typical known blow molding procedure involves the creation of a parison, confinement of the parison within a cavity, blowing of the parison in successive stages to finally yield a completed article conforming to the shape of the cavity, and finally removing the blow molded article after it has become substantially dimensionally stable.
The present invention arises in part through the recognition that the procedures of prior blow molding of smaller articles are not readily conformable to the blow molding of large thermoplastic articles, such as large drums.
One problem which is encountered in the blow molding of large plastic articles is that a substantial amount of cooling must take place in order for the plastic material to become dimensionally stable. This can be appreciated when one considers the fact that as the size of an article increases, its surface area increases at a lower rate than does its volume. This means that cooling efficiency of a blow molding machine decreases as a function of article size, and the problem is compounded because larger volume articles, such as drums for carrying bulk material, require larger wall thicknesses to provide strength.
Also, in the case of drums such as those referred to above, the users of such drums may require threaded holes in the top wall of the drum. Thus, the incorporation of threaded holes in a drum must also be addressed if a blow molded drum is to be commercially acceptable.
The present invention is directed to a new and unique blow molding machine which is capable of efficiently blow molding large thermoplastic articles such as cylindrical drums and which is endowed with the capability of molding threaded holes into the top wall of such drums when such threaded holes are required.
Briefly, the present invention comprises a multi-station blow molding machine which, in its preferred embodiment as disclosed herein, has a rotary table on which mold cavities are disposed for advancing rotation about a central vertical axis. There are four such stations 90.degree. apart, and there are four mold cavities on the rotary table also 90.degree. apart. The first station is the one at which the blow molding of an article commences, and it is at this same station that completed articles are removed after they have passed through the intervening stations.
At the first station an article is blow molded substantially to its final shape against the walls of the cavity and after this step has been completed the mold is advanced to the second station.
Associated with each cavity is a blow pin assembly and it is via the blow pin assembly that the blow medium is introduced into the cavity to blow the article.
The blow pin assembly is cooperatively associated with the cavity and moves along with the cavity from station to station.
Associated with the first station is a mechanism for operating each cavity between open and closed positions. The illustrated form of mold cavity and mechanism comprises the cavity being in two halves which are pivotally mounted on a vertical shaft on the table spaced radially of the central vertical axis of the table. The two cavity halves open radially outwardly of the central vertical axis in a clamshell-like fashion and are operated by toggles. Drive means for operating the toggles are selectively engagable with the toggles at the first station to open and close the cavity. After the cavity has been closed the drive means are disengaged from the toggles to permit the table to be rotated 90.degree. to advance the cavities. As a cavity travels through the stations from the first station through the second, third and fourth stations and back to the first station, the toggles are effective to hold the halves of the cavity closed.
At the first station a blow assembly is mounted on a movable carrier which takes the form of a wheeled cart. The cart is operable to transport the blow assembly radially inwardly and outwardly relative to the central vertical axis of the table such that when the cart is underlying a mold cavity the blow assembly may be operatively connected with and disconnected from the cavity. When a new article is to be molded, the blow assembly supports the blow pin assembly and the two are elevated in unison on the cart to a position where the blow pin assembly may be cooperatively associated with the cavity. Cooperative association takes place when the two halves of the cavity are closed onto the blow pin assembly. Thereafter, the blow assembly is operated to introduce into the cavity, via the blow pin assembly, blowing medium for blowing an article to substantially its final shape in the cavity. The raw material for the article is provided by an associated extruder and feeder which introduces a parison into the cavity prior to the blow. After the blow has been completed, the blow assembly is lowered onto the cart to disconnect from the blow pin assembly. When the blow assembly clears the blow pin assembly, the table can rotate to advance the cavities.
Upon arrival of a cavity at the second station, an auxiliary blow assembly is cooperatively associated with the blow pin assembly of the cavity to exhaust the blow medium which was introduced into the cavity to blow the article at the first station and to introduce new medium into the cavity. This promotes the proper curing of the thermoplastic material and may involve the introduction of different types of media to promote different curing characteristics. At the same time as the auxiliary blow assembly is operating on the cavity at the second station, a new article is being blown in a cavity at the first station.
After the second station, a cavity is advanced to a third station which also contains an auxiliary blow assembly which operates on the molded article contained therein in similar manner to that of the second station. This promotes a further cooling effect on the molded plastic article leading toward its final dimensionally stable form.
The fourth station also contains an auxiliary blow assembly which operates on the article contained in a cavity at the fourth station in similar manner to that of the third and second stations. It will be appreciated that while a given operation is being conducted at any given station, the other stations are concurrently performing their respective functions.
After the fourth station, the cavity returns back to the first station where it is opened and the completed article removed.
A completed article arriving at the first station must be removed from its cavity before a new article can be molded in that cavity. Removal of a completed article is accomplished by the cart and the blow mechanism. The blow mechanism is once again elevated on the cart to engage the blow pin assembly on the cavity containing the completed article. The cavity halves are opened to allow the completed article to clear the cavity and the cart is moved radially outwardly to transport the article on the cart via the blow pin assembly and the blow assembly.
Where the article is a cylindrical walled drum having holes in its top wall, the article is molded upside down so that the top wall of the article is at the bottom of the cavity. It is at the location of the holes that the blow pin assembly is engaged with the article. Where the top wall of the article has threaded holes, the threaded holes are formed by the molding of the article around complementary threaded portions of the blow pin assembly.
The blow pin assembly and the blow assembly are provided with means for rotating these threaded portions about their axes so as to unthread the threaded portions of the blow pin assembly from the completed article. Thus, the threaded portions are rotated at the time of removal of the article from the machine so that the completed article contains the desired threaded holes. After the completed article has been removed from the blow pin assembly, the blow pin assembly is carried by the cart radially inwardly for cooperatively re-association with the mold cavity preparatory to the blow molding of a new article in the cavity. The procedure thereafter repeats itself in the same manner as described above.
The foregoing features, advantages and benefits of the invention, along with additional ones, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a preferred embodiment of the invention according to the best mode contemplated at the present time in carrying out the invention.